The present invention relates to a threshold value operation circuit which performs a threshold value operation on a level of an input signal, and generates an AC (alternating current) signal when at a set level (corresponding to logic value 1), and does not generate an AC signal when not at the set level (corresponding to logic value 0). In particular the invention relates to a threshold value operation circuit which can perform an analogue type threshold value operation, and which can generate an AC signal of stabilized frequency and duty ratio when an input signal is a set level, and to an AND gate circuit, a self hold circuit, and a start signal generating circuit to which this threshold hold value operation circuit is applied.
With signal processing in safety equipment and the like, where fail-safe characteristics are required, the presence of an AC signal is processed as a logic value 1 and the absence of an AC signal is processed as a logic value 0. Finally a threshold value operation is carried out on the AC signal and a judgement is made as to whether or not this is within a predetermined level, and a signal indicating safety or danger is output.
As one example of a conventional threshold value operation circuit applicable to such fail-safe signal processing, there is for example a fail-safe window comparator/AND gate. The circuit construction and operation of this is shown in for example Trans. IEE of Japan, Vol. 109-C, No. 9, September 1989 under the heading xe2x80x9cA Structural Method for an Interlock System using a Fail-safe Logic Element having Window Characteristicsxe2x80x9d. Moreover, this is also shown in the specifications of U.S. Pat. No. 4,661,880, U.S. Pat. No. 5,027,114, and in the disclosure of Examined Japanese Patent Publication No. 1-23006.
Representative circuit examples thereof are shown in FIG. 31(A) and FIG. 31(B).
FIG. 31(A) is a window comparator having a level detection function. When a level LI of an input I is in a range (r1+r2+r3)E/r3 less than L1 less than (r4+r5)E/r5 (where E is the circuit power source voltage, and r1xcx9cr5 are the resistance values of resistors R1xcx9cR5), the circuit self oscillates with respective transistors Q1xcx9cQ3 repeating an on/off operation in a process of Q2: OFFxe2x86x92Q3: OFFxe2x86x92Q1: ONxe2x86x92Q2: ONxe2x86x92Q3: ONxe2x86x92Q1: OFFxe2x86x92Q2: OFFxe2x86x92 . . .
Furthermore, FIG. 31(B) is a circuit having a level detection function and an AND function, with two window comparators as shown in FIG. 31(A) connected in cascade. When the levels LI1, LI2 of the inputs 11, 12 are respectively in the range of (r1+r2+r3)E/r3 less than LI1 less than (r4+r5)E/r5, and (r1xe2x80x2+r2xe2x80x2+r3xe2x80x2)E/r3xe2x80x2 less than LI2 less than (r4xe2x80x2+r5xe2x80x2)E/r5xe2x80x2 (where E is the circuit power source voltage, and r1xe2x80x2xcx9cr5xe2x80x2 are the resistance values of resistors R1xe2x80x2xcx9cR5xe2x80x2), the circuit self oscillates , and if even one of L1 and LI2 goes outside of the abovementioned range, oscillation stops.
Furthermore, as another threshold value operation circuit, there is for example the one disclosed in Japanese Unexamined Patent Publication No. 8-162940, previously proposed by the present applicants. This is shown in FIG. 32.
With the circuit of FIG. 32, if an output X2 from an envelope detection circuit A based for example on an AC signal VIN exceeds a power source voltage Vcc, then due to a switching (on/off) of a photocoupler PC1 which receives an AC signal from a separately provided AC source, a switching current intermittently flows in a photodiode of a photocoupler PC2 via a current decreasing resistor R. Hence a phototransistor of the photocoupler PC2 switches (on/off), and an AC signal is generated (output is logic value 1). When an AC signal VIN is not input to the envelope detection circuit A, the output X2 from the envelope detection circuit A becomes approximately the power source voltage Vcc. Hence even if the photocoupler PC1 switches, a switching current does not flow to the photocoupler PC2, and an AC signal is not generated (output is logic value 0). The envelope detection circuit B voltage doubler rectifies the AC signal due to the switching of the phototransistor of the photocoupler PC2, and outputs a DC (direct current) signal Y which is larger than the power source voltage Vcc.
However, the threshold value operation circuit which uses the window comparator of FIG. 31 is a self-oscillation circuit, and the oscillation frequency and duty ratio are approximately determined by the on and off delay time of the respective transistors. Therefore due to changes in the characteristics of the transistors resulting from variations in transistor characteristics, temperature and humidity and the like, or due to the input signal level, the oscillation frequency and the duty ratio of the AC output change, so that there is the problem in that a stabilized AC output cannot be obtained. Moreover, this threshold value operation circuit uses processing outside of the power source limits (when an input signal of a predetermined level greater than the circuit power source voltage is input, an AC signal is generated), and must satisfy relationships where the input signal level and the threshold value of the window comparator both change in proportion to the circuit power source voltage. Therefore, there is the restriction that the input signal oscillation circuit and the window comparator must use the same power source. Hence the degree of freedom for circuit layout is low.
On the other hand, with the threshold value operation circuit of FIG. 32, since the construction is such that the AC signal from an external AC source is used to give oscillation, the oscillation frequency and the duty ratio are not susceptible to influence from the characteristics of the circuit elements or the input level, and hence a stabilized AC signal can be obtained. However, this circuit confines itself to judging as to whether or not the input signal is greater than the circuit power source voltage Vcc, in other words to digital judging for the presence or absence of the input signal, and does not have an analogue threshold value detection function for the input signal.
The present invention takes into the consideration the above situation, with the object of providing a threshold value operation circuit, which can perform an analogue threshold value operation, and which can generate an output AC signal of stabilized frequency and duty ratio, and for which the circuit layout degree of freedom is high. Moreover, it is an object to provide an AND gate circuit, a self hold circuit and a start signal generation circuit to which this threshold hold value operation circuit is applied.
Accordingly, with the present invention, there is provided a threshold value operation circuit which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level.
More specifically, the construction may be such that the input signal subject to threshold value operation is input to the modulation device, and a modulation signal modulated by the modulation device is input to the level detection device, and the level detection device generates an AC signal when the modulation signal is a set level. Furthermore, the construction may be such that the input signal subject to threshold value operation is input to the level detection device and level detected, and a signal which has been level detected by the level detection device is modulated by the modulation device. Moreover, the construction may be such that there is provided an AC signal generating device for generating an AC signal which is switched by input of a modulation signal modulated by the modulation device.
With such a construction, an analogue type threshold value operation becomes possible with respect to the input signal subject to threshold value operation. Furthermore, an AC output which is synchronized with a switching signal from outside can be obtained, so that an AC output of stabilized frequency and duty ratio which is not influenced by characteristic changes and the like of the circuit elements can be obtained.
With the modulation device the construction may be such that a switch device is switched by the switching signal from the AC source, to modulate an input signal. In this case, if the switch device is a photocoupler which is switched by input of the switching signal from the AC source, then the AC source side and the input side of the level detection device can be isolated. Moreover, if the switch device is a transistor, then switching at high speed becomes possible.
In the case where a transistor is used in the switch device, the switching signal may have an amplitude of a range between the input signal and the voltage of the constant voltage supply line, and a signal which does not cross over the threshold value of the level detection device may be used for the switching signal input to the transistor. With such a construction, then even if with a fault in the transistor, the switching signal is input directly to the level detection device, the problem where an AC signal is generated from the level detection device by the switching signal regardless of the input signal does not arise.
The construction may be such that the modulation device incorporates a buffer circuit which takes the input of the AC source switching signal, and supplies this to an input line to which the input signal is applied.
In the case where the level detection device comprises a comparator, the construction may be such that the AC output from the comparator is transmitted to an output circuit side via a photocoupler. With such a construction, the respective power supplies for the output circuit side and the threshold value operation circuit side can be made independent, so that the degree of freedom for the layout of the threshold value operation circuit is increased.
Furthermore, the level detection device constructed to incorporate a Zenner diode and a photocoupler for generating an AC signal which is switched by an output from the Zenner diode. In the case of this construction, the degree of freedom for the layout of the threshold value operation circuit is increased.
The threshold value operation circuit of the present invention comprises: a first Zenner diode for level detecting an input signal subject to threshold value operation; a switch device which is switched by a switching signal from an AC source, for modulating an output signal from the first Zenner diode; a first photocoupler which is switched by input of a modulation signal modulated by the switch device, for modulating the input signal subject to threshold value operation; a second Zenner diode for level detecting a modulation signal of the input signal subject to threshold value operation which has been modulated by the switching operation of the first photocoupler; and a second photocoupler which is switched by an output signal from the second Zenner diode, for generating an AC signal, and when the level of the input signal subject to threshold value operation is a set level, an AC signal is generated from the second photocoupler.
With such a construction, since the upper limit threshold value and the lower limit threshold value can be set independently, the generation of an AC output when the Zenner diode is short-circuited can be prevented.
An AND gate circuit which uses a threshold value operation circuit of the present invention comprises a plurality of threshold value operation circuits connected in cascade each of which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level, wherein an AC output from a former stage threshold value operation circuit is input as a switching signal to a modulation device of a latter stage threshold value operation circuit, instead of an AC source.
Another AND gate circuit of present invention comprises: a threshold value circuit, and outputs the added values for all of the AC input signals fro operation circuit which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level; and an adder circuit comprising a plurality of rectifying circuits for respectively rectifying AC input signals, which sequentially adds a rectified output from a first stage rectifying circuit to a rectified output from a next stage rectifying circuit, and outputs the added values for all of the AC input signals from a final stage rectifying circuit, wherein the addition output from the adder circuit is supplied as the input signal subject to threshold value operation of the threshold value operation circuit.
A self hold circuit of the present invention which uses a threshold value operation circuit comprises two threshold value operation circuits each of which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level, wherein one threshold value operation circuit is made a trigger circuit and the other threshold value operation circuit is made a hold circuit, and an AC signal which is generated based on a logical sum operation on an output from the trigger circuit and an output from the hold circuit is input to a modulation device of the hold circuit as a switching signal instead of an AC source.
Moreover, another self hold circuit of the present invention comprises: a threshold value operation circuit which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level; and an adder circuit comprising two rectifying circuits for respectively rectifying AC input signals, which adds a rectified output from a former stage rectifying circuit to a rectified output from a latter stage rectifying circuit, and outputs the added values of the two AC input signals from the latter stage rectifying circuit, wherein the addition output from the adder circuit is supplied to an input terminal of the threshold value operation circuit as the input signal subject to threshold value operation, and an input terminal of the latter stage rectifying circuit of the adder circuit is made a hold input terminal, and an input terminal of the former stage rectifying circuit is made a trigger input terminal, and the AC output signal from the threshold value operation circuit is rectified and then fad back to an output terminal of the former stage rectifying circuit.
A start signal generating circuit of the present invention which uses a threshold value operation circuit comprises: a start switch comprising a first contact point and a second contact point having a complimentary relation to each other for on/off switching; a capacitor which is charged via the first contact point which comes on when the start switch is pressed, and which is discharged via the second contact point which comes on when the start switch returns, for generating an output signal; and a threshold value operation circuit which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level; wherein the output signal from the capacitor is made the input signal subject to threshold value operation of the threshold value operation circuit, and the output from the threshold value operation circuit is made a start signal for a machine or the like.
With such a construction, only when the two contact points of the start switch are operating normally, and the start signal level is a set level, is an AC signal generated from the threshold value operation circuit, so that a start signal for a machine or a like can be generated.
Another start signal generating circuit of the present invention comprises a first and a second threshold value operation circuits each of which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level, wherein an AC output from the first threshold value operation circuit is input to the modulation device of the second threshold value operation circuit as a switching signal instead of the AC source, and wherein a power source voltage level is subject to a threshold value operation by the first threshold value operation circuit, and an output signal which is generated based on an ON operation of an operation button is made the input signal subject to threshold value operation of the second threshold value operation circuit, and the output from the second threshold value operation circuit is made a start signal for a machine or the like.
Another start signal generating circuit of the present invention comprises a self hold circuit provided with two threshold value operation circuits each of which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level, wherein one threshold value operation circuit is made a trigger circuit and the other threshold value operation circuit is made a hold circuit, and an AC signal which is generated based on a logical sum operation on an output from the trigger circuit and an output from the hold circuit being input to a modulation device of the hold circuit as a switching signal instead of an AC source,
wherein an output signal generated in accordance with the on/off operation of a start switch comprising first and second contact points having a complimentary relation to each other for on/off switching, is input to the trigger circuit of the self hold circuit as the input signal subject to threshold value operation, and only when the input signal subject to threshold value operation is a set level, and the input signal level to the hold circuit is a set level, is a start signal generated from the self hold circuit.